Spacecraft, such as geosynchronously orbiting three axis stabilized satellites, typically carry some type of payload, such as an Earth imaging camera and related equipment. The related equipment can include a stabilized gimbal system for pointing the camera relative to the body of the spacecraft. Satellites intended for meteorological purposes are one well known example.
It can be appreciated that it is important to maintain the payload in a desired pointing direction with respect to the Earth, and to thus compensate for movements of the satellite away from a desired orbital configuration.
Although the satellite typically contains a closed loop control system to maintain the satellite's orbital attitude within predetermined limits, such control systems typically operate with a fast time constant. That is, a deviation from the satellite's desired attitude as sensed by, for example, one or more of Earth sensors, star sensors, solar sensors, gyroscopes, etc., is quickly corrected by energizing a spacecraft actuator, such as a thruster. However, the speed of the control Loop implies a higher noise level, relative to a control loop that has a longer intrinsic delay and slower time constant. As such, the presence of the higher noise level makes the spacecraft's attitude control system less than desirable for use in controlling the attitude of the payload, especially if the payload is required to obtain accurately registered images of the Earth or some other object.